Myoglobin saturation is measured in single cells of dog gracilis muscles at rest and during phasic exercise under various conditions of O2 supply. Frequency distributions of saturations and corresponding O2 partial pressures are used to characterize the performance and control of the O2 delivery system. Capillary recruitment and decruitment is studied during graded exercise to learn a) relation between controls of flow and capillary density and b) the quantitative contribution of recruitment to O2 delivery in exercise.